This invention constitutes an improvement over the propeller pitch change mechanism disclosed in U.S. Pat. No. 5,199,850 granted to E. H. Kusiak and myself on Apr. 6, 1993 entitled "Pitch Stop Assembly for Variable Pitch Propulsor" which is commonly assigned to United Technologies Corporation. In that patent the pitch change is effectuated by a ballscrew and ballscrew nut assembly that is responsive to a mechanical differential gearing that imparts rotary motion to the ballscrew that, in turn, translates the ballscrew nut. The ballscrew nut is attached to the root of the propeller blade through a yoke, trunnion and a connecting link for increasing and decreasing the pitch of the blade. Each blade includes the trunnion and connecting link and are moved in unison so that the blade angles are uniformly changed.
Of necessity, the pitch change mechanism disclosed in the U.S. Pat. No. 5,199,850, supra, includes an anti-torque tube assembly that permits axial motion while preventing rotational movement of the ballscrew nut. The heretofore utilized anti-torque tube or sliding anti-rotation spline which comprise mating splines formed in a cooperating pair of concentrically mounted cylinders supported around the engine's center line serves to ensure that the ballscrew nut will not rotate as a result of the momentum of the ballscrew and by the force generated by the centrifugal twisting moment of the blades. As one skilled in this art will appreciate the anti-rotation spline assembly is not only heavy but is an expensive part of the pitch change assembly.
I have found that I can eliminate the individual connecting links and the anti-rotation spline assembly by virtue of this invention. This invention contemplates extending the trunnion attached to the root of each blade that cooperates with an arm and pin assembly that is operatively connected to the ballscrew nut. By virtue of this invention, pitch change movement is effectuated in such a manner that the blades will not bind up and the ballscrew nut is allowed to slightly rotate. To this end, the arm end, in one embodiment, is made to translate rectilinearly relative to the pin and in an alternative embodiment the pin translates relative to the trunnion end. In another embodiment, a translating cylinder operatively connected to the blades for pitch change movement is designed so that the cylinder is allowed to slightly rotate in a manner that eliminates the need of the sliding anti-rotation spline. In another embodiment that utilizes the translating ballscrew nut, the dimension of the yoke arms are judiciously selected to allow minimal bending in the plane that will assure that the blades do not bind up upon pitch change movement.
This invention is particularly efficacious in the advanced ducted fan (ADP) concept of a gas turbine engine. The ADP is a new propulsor design that affords extremely high bypass ratios for attaining fuel economy and thrust reversing so as to eliminate the heavy and complex engine thrust reversing mechanism. As one skilled in this art will appreciate, the weight, envelope size and economy of the variable pitch fan or propeller is extremely important and the specification is typically mandated by the engine manufacturer so that the propeller designer and manufacturer is confined to these specifications. This, of course, places a heavy burden on the propeller designer. Thus, a gas turbine engine with an ADP affords all the advantages that are available in the propeller technology, including, but not limited thereto, the flow optimization for all aircraft operating modes, feathering in the event of a malfunction, low pitch stop, pitch lock and reversing. As mentioned above, these features are attainable and desirable only if the propeller fall within the constraints of the specification established for a particular engine.
In the context of this patent application the terms "propeller", "ADP" and "propulsor" are used interchangeably inasmuch as the concept of this invention is applicable to propulsors that may be in the propeller class or ADP class.